Misfire at higher RPM's

[Bob A.]

Hi All,

I have a completely restored 327/340 engine for a 63 Roadster. On the dyno, the engine had occasional misfires at +/- 5500 rpms. At lower rpms it runs fine. All settings have been checked, carburation-timing... Any guidance on potential causals or diagnostics to correct misfire are appreciated.

Bob

[Gary C.]

Re: Misfire at higher RPM's

Bob,

Check the distributor vacuum advance and point float if the fuel pump pressure to the carb is okay.

Gary
....

[Leif A.]

Re: Misfire at higher RPM's

Ignition coil going bad?

[Bob A.]

Re: Misfire at higher RPM's

Fuel pump pressure and vacuum advance OK. Will replace points and verify point float.

Thx Gary

[Bob A.]

Re: Misfire at higher RPM's

Leif,

Will check coil as well.

Thx,
Bob

[Domenic T.]

Re: Misfire at higher RPM's

Bob,
Had the wrong range plugs do that to mine, + a dirty plug. Also If it was driven easily the large 202 intake can have a buildup of baked oil that will add weight and cause a valve float. Just adding to the good suggestions on the other posts. I bought a 65 SS in San Diego and had to drive about 125
miles home. It was night and when i hit i it misfired about 5,000. In the rear view mirror I was looking to see if it smoked, and I saw glowing embers coming out of the exhaust. It seemed to clear up a bit, then I used GM top cleaner and that just about eliminated it and 6,000 and under had no problem.

Dom

[Duke W.]

Re: Misfire at higher RPM's

In the early days my SWC's 340 HP ignition was constantly breaking up as low as 4500 revs, and it had an idle stability issue. The later was due to the OE 201 15 VAC not passing the Two-Inch Rule that I replaced with a 236 16 (current B28). I eventually solved the former by doing a "blueprint overhaul" of the distributor... shimmed up the end play to the spec in the '63 Corvette Shop Manual, installed the high breaker arm tension points, and replaced the breaker plate due to it fitting loose and wobbling.

You can probably confirm if your distributor has similar issues by free revving it and watching for dwell variation, which should no more than 2 degrees at 6500.

Also make sure the spark plugs are AC heat range 5, preferably non-resistor or equivalent in another brand like NGK B4 and Denso W14-U.

Both here and on the Corvette forum I have articles explaining how to do a distributor blueprint overhaul and how to select the proper point set for the engine configuration. Search either for threads started by me. There aren't many.

Duke

[Frank D.]

Re: Misfire at higher RPM's

It used to be fairly common for a high RPM miss to be due to faulty or deteriorated plug wires...
In addition arcing inside the distributor cap would sometimes cause it.

Observing the engine running at high RPM in the dark might reveal some spark leak.

[Danny P.]

Re: Misfire at higher RPM's

Hi All,

I have a completely restored 327/340 engine for a 63 Roadster. On the dyno, the engine had occasional misfires at +/- 5500 rpms. At lower rpms it runs fine. All settings have been checked, carburation-timing... Any guidance on potential causals or diagnostics to correct misfire are appreciated.

Bob

Bob, check dwell i set them up at 28 Degree, check sparks plugs see which one is leaning out, coil heating up and breakup at high RPM

[Bruce W.]

Re: Misfire at higher RPM's

Bob, I had the exact same problem with my 62 340 hp. tried everything. new coil, plugs, had the distributor and carburetor rebuilt. adjusted the timing and valves. Still would break up at 5,000Rpm. As a last resort I changed the plug wires with K&B wires. that was the problem all along. I hope I saved you some unneeded aggrevation.
Bruce